Field of the Invention
The invention relates to a pressurized-water reactor having a multiplicity of fuel assemblies being mutually adjacently disposed in the interior of a pressure vessel on a core support at the bottom of the pressure vessel, each of the fuel assemblies containing a bundle of fuel rods disposed around control-rod guide tubes and being supported at apertures in a grid plate by a top carrying a top plate covering the bundle, a plenum being formed above the grid plate inside the pressure vessel, attachments protruding into the plenum at the upper surface of the grid plate, the plenum having a lateral outlet, and the pressure vessel having a device for deflecting a coolant flow from an inlet into the pressure vessel through the core support, for distributing it over the individual fuel assemblies and for guiding it along the fuel rods, through passage openings in the top plates of the fuel assemblies and through apertures in the grid plate into the plenum.
Fuel assemblies of pressurized-water reactors contain a bundle of fuel rods which are disposed around control-rod guide tubes, with the bundle of fuel rods being covered by a top plate in the top of the fuel assembly. Inside a pressure vessel, a multiplicity of such fuel assemblies are disposed adjacently on a core support at the bottom of the pressure vessel and are supported with their top parts on the apertures of a grid plate. Formed above the grid plate inside the pressure vessel is a plenum into which attachments on the upper surface of the grid plate protrude. The plenum contains a lateral outlet, such as one or more outlet nozzles for a coolant flow. The coolant flow is guided by appropriate devices from an inlet in the pressure vessel to the core support at the bottom of the pressure vessel, is distributed there over the individual fuel assemblies, then flows along the fuel rods and emerges through passage openings in the top plates of the fuel assemblies in order to enter the plenum through the apertures in the grid plate.
In this case, the core support at the bottom of the pressure vessel can already carry throttle plates or inserts in order to distribute the coolant flow uniformly over the entire cross section of the pressure vessel. The interstices between the fuel rods and the guide tubes of a fuel assembly are connected to one another and to the interstices between adjacent fuel assemblies so that coolant flows directed transversely to the fuel assemblies can occur in the pressure vessel. This may be desirable in order to achieve thorough mixing between hotter and cooler regions of the coolant, for which purpose spacers with appropriate deflection devices may be provided on different axial planes of the fuel assembly. Such spacers are required in any case in order to fix the lateral spacing of the fuel rods. However, apart from such spacers supporting the fuel rods, their own grid structures may also be provided additionally to hold further mixing devices on the fuel assemblies.
The attachments which protrude from the upper surface of the grid plate into the dome of the pressure vessel (that is to say the plenum for the coolant heated on the fuel rods) are necessary, for example, to support the grid plate mechanically and to receive the control rods which can be introduced into the control-rod guide tubes. The partial flows of the coolant, into which the coolant is divided at the lower core support and which emerge after flowing through the individual fuel assemblies through the individual apertures in the grid plate, therefore have to overcome an individual flow resistance on their path to the outlet, which flow resistance is determined by the length of the respective flow path and the obstacles disposed in that path. A pressure thus occurs in the coolant when passing through the passage openings in the plates, which pressure is distributed inhomogeneously over the cross section of the pressure vessel. When passing through the top plates, the coolant thus suffers damming-up which, due to the geometrical configuration of the attachments on the top plate and of the lateral outlet, may be different for each fuel assembly and already leads to pressure differences and resultant transverse flows in the axial zone of the pressure vessel in which the fuel rods are seated.
The pressure differences are already one of the causes of bending of the fuel rods and fuel assemblies. Additionally, the transverse flows cause the fuel rods and the structural elements of the fuel assemblies to vibrate and to be subjected to mechanical loading. In total, other physical loads on the fuel assemblies are thus intensified, in such a way that damage may occur on the fuel assemblies.
In order to avoid horizontal pressure differences and transverse flows, the coolant can be guided vertically through the plenum in the dome of the pressure vessel and conducted away through corresponding vertical outlet nozzles which take into account the geometry of the attachments on the grid plate. However, that leads to a complicated construction or an impermissible structural height of the pressure vessel.